1. Field of the Invention
This invention relates to heteropoly acid catalyst compositions, a method of making such catalyst compositions and a process for the oxidation of unsaturated aldehydes, such as methacrolein, to unsaturated carboxylic acids, such as methacrylic acid, in a vapor phase reaction using such catalyst compositions.
2. Description of the Prior Art
Various catalysts are known for the gas phase catalytic oxidation of unsaturated aldehydes to unsaturated carboxylic acids. Included are molybdenum-based mixed metal oxides compound which can contain oxides of metals such as phosphorus, arsenic, cesium, rubidium, cobalt, nickel, iron, chromium, antimony, tellurium and silicon in addition to molybdenum. These same metals and others may occur in heteropoly acid compounds as metal oxide clusters forming heteropolyoxoanions in acid form instead of simple metal oxides. Heteropoly acid compounds are also known as catalysts for the gas phase catalytic oxidation of unsaturated aldehydes to unsaturated carboxylic acids. Heteropoly acid compounds have a central metal atom surrounded by a framework of other metal atoms connected to each other and the central metal atom through oxygen atoms. The central metal atom is different (“hetero”) from the framework metal atoms.
One problem of heteropoly acid compounds as catalysts is thermal stability. In process operating temperatures, the heteropoly acid compound catalysts tend to deactivate by chemically and physically breaking down. Commercially useful heteropoly acid compound catalysts compound catalysts must have thermal stability for acceptable catalyst lifetime.